Known in the art is a radio frequency accelerator for accelerating ions, electrons, or other charged particles. A radio frequency accelerator is provided inside it with an acceleration cavity for accelerating charged particles. The acceleration cavity forms a resonance circuit and has a unique resonance frequency. By supplying radio frequency power in accordance with this resonance frequency from the outside, a radio frequency electric field is excited inside of the acceleration cavity. A radio frequency accelerator can accelerate charged particles up to a desired energy by injecting charged particles at a predetermined timing in the state where the radio frequency electric field is excited.
Radio frequency accelerators are classified by the shape of the path of the charged particles into linear accelerators (linacs) and circular accelerators. Linear accelerator is accelerator with straight paths of the charged particle beam, while circular accelerators are accelerators with curved paths of the charged particle beam. Linear accelerators include, for example, radio frequency quadrupole accelerators, drift tube accelerators, etc.
A radio frequency quadrupole accelerator is provided with four electrodes. The four electrodes form two mutually facing pairs. At the tips of the electrodes, wave shapes suitable for acceleration of a beam are formed in the direction of the acceleration beam axis. In the space surrounded by the four electrodes, an electric field is formed for accelerating and focusing a beam. By injecting charged particles into this space, the charged particles are accelerated.
Japanese Patent Publication (A) No. 5-62798 discloses an external resonance type quadrupole particle accelerator which is provided with an accelerator tube which has electrodes forming the quadrupole structure inside of it and a radio frequency resonance circuit which supplies a resonance voltage to the electrodes. This publication discloses the radio frequency resonance circuit being comprised of a capacitor and two coil conductors serving as an inductance member and the inductance member being arranged in series between the capacitor and the accelerator. According to this accelerator, the resonance frequency can be made to change over a broad range and, further, the quality factor can be raised.